Method for the purification of potassium carbonate



Feb. 26, 1963 G. M. BRUNNER ETAL METHOD FOR THE PURIFICATION oF Po'rAssIuM cARBoNATE Filed March 19. 1959 Fille as Cool Float lI .Pecanz Jolzd zgzzzk George M. Brunner William V Hauck Albert Adams 3,079,227 METHD FR THE PUREECATKN F iPUiCSSlUll/i CAi BNATE George M. Brunner and William V. Hauch, Niagara Falls,

NX., and Albert Adams, Qarlsbad, N. Men., assignors to International l'ififlnerals Chemical Corporation, a

corporation of New Yorlr Filed Mar. i9, 1959, Ser. No. 800,527 3 Ciairns. (Cl. 23--63) The present invention relates to the puriiication of potassium carbonate. More particularly, it relates to the purification of potassium carbonate containing potassium chloride and other impurities.

Cure method for the production of potassium carbonate comprises electrolysis of aqueous potassium chloride solution to form potassium hydroxide containing a small proportion of potassium chloride, followed by carbonation of the aqueous potassium hydroxide solution, suitably with liue gas, to form potassium carbonate containing a small proportion of potassium chloride as the major impurity.

One method ot purifying potassium carbonate containing potassium chloride as impurity is repeated crystallization of the salt from aqueous solution until the desired purity is attained. This can be achieved by dissolving the salt in hot water and recrystallizing by evaporation and/or cooling. Such a procedure not only results in poor yields of puritied product, but is timeconsuming aud costly.

An object of the present invention is to provide an improved process for the purification of potassium carbonate.

Another object is to provide an integrated process for the purilication of potassium carbonate wherein all materials used in the process are substantially completely recovered.

Another object is to provide an improved process for the separation of potassium chloride and other impurities from potassium carbonate produced from electrolytic potassium hydroxide, containing potassium chloride as impurity.

Other objects and advantages of the present invention will become apparent upon a fuller understanding of the invention as hereinafter set forth.

Generally speaking, the present invention effects puriiication of potassium carbonate by a technique which may involve (l) selective leaching with water under controlled conditions, (2) centrifugal washing of the solid phase, and (3) concentration of the leach liquor to effect crystallization, followed (4) by flotation of the resulting slurry and (5) appropriate recycle of the oat and sink fractions.

The invention is applicable to the purification of potassium carbonate containing a small proportion of potassium chloride as impurity (e.g., up to about 5% by weight of the total solids, dry basis). The starting material is slurried in water (unless already existing in the form of a slurry) and is adjusted to an elevated temperature between about 60 C. and the boiling point, preferably between about 80 and about 100 C. The proportion of starting material to water should be suiiicient to yield a slurry containing between about and about 70% by weight of undissolved solids at equilibrium. The mixture is agitated, preferably until equilibrium has been approached or reached, and is then filtered, centrifuged, or decanted to separate the liquid and solid phases. rPhe liquid phase is an aqueous solution substantially completely saturated with potassium carbonate and 0rdinarily at or near saturation with potassium chloride, while the solid phase is potassium carbonate containing potassium chloride in a proportion to potassium carbonate substantially lower than in the liquid phase.

Inasmuch as the solid phase occludes a quantity of the liquid phase, which contains a small proportion of dissolved potassium chloride, the solid phase may be further puritied by selective leaching under controlled conditions. To carry out the leaching, the solid phase is first slu-rried in a wash medium, suitably water or preferably an aqueous solution saturated with essentially pure potassium carbonate. The proportion of undissolved solids to the wash medium should be sucient to provide a slurry containing between about 20% and about 35% by weight of undissolved solids. The resulting slurry is centrifuged to separate the solid and liquid phases. The liquid phase, containing ordinarily up to about or more of the potassium chloride occluded in the solid phase prior to the washing step, together with a high proportion of potassium carbonate, is recycled to the original aqueous slurry of impure potassium carbonate. The washed solid phase is a purified potassium carbonate product of reduced chloride content, ordinarily less than about 0.2% rand in many cases less than about 0.1%.

Depending on the proportion of chloride as impurity contained in the original potassium carbonate and/or the desired purity of the iinal product, it may be Iadvantageous to again subject the washed potassium carbonate to the selective leaching process generally according to the same procedure and under the same conditions as described hereinabove.

The liquid phase from the original aqueous slurry of impure potassium carbonate is substantially completely saturated with potassium carbonate and potassium chloride at the elevated temperature of separation from the solid phase. The potassium values therein can advantageously be recovered in the following manner. The liquid phase can optionally be evaporated to higher concentration if desired. lt is then cooled to a tempera-ture sufficiently low to produce crystallization of salts therefrom. For this purpose, the cooling is ordinarily carried to room temperature or somewhat below, the precise temperature being chosen to effect maximum crystallization of salts while avoiding production of a slurry which is too thick for further processing. By this means, a major.

proportion of the potassium chloride content of the liquor crystallizes in combination with a substantial proportion of potassium carbonate.

The resulting slurry is subjected to flotation in a conventional flotation cell, whereby the potassium chloride is selectively floated therefrom, preferably, without the use of a conditioning agent. lf desired, a cationic flotation reagent such as a long-chain fatty acid amine (e.g., distilled octadecylamine acetate) may be employed; however, such reagents tend to cause foaming and dissolution of a portion of the solid potassium chloride, and their use is not generally preferred.

The float product is withdrawn, washed to remove potassium carbonate, and recycled for production of additional potassium hydroxide by electrolysis under conventional conditions. The sink fraction, containing a major proportion of the potassium carbonate originally present in the flotation mixture, is recycled to the original aqueous slurry of impure potassium carbonate.

For a better understanding of the present invention, reference may be had to the drawing which illustrates a preferred embodiment of the present invention.

In a preliminary operation, an aqueous solution of potassium chloride is electrolyzed to produce an aqueous solution of potassium hydroxide containing a small proportion of potassium chloride (ordinarily around 15% of the solution) as impurity. The resulting solution is concentrated by evaporation, preferably to about 45% potassium hydroxide by weight, whereby all but about 1% of the potassium chloride crystallizes. The potassium chloride crystals are separated from the solution by centrifugation. The liquid phase is treated with ue gas, comprised essentially'of carbon dioxide, nitrogen, andwater vaporyto convert the potassium hydroxide therein into potassium carbonate.

4The resulting solution of potassium carbonate, containingiles's Vthanabout 1% potassium` chloride, dry basis, is subjectedto evaporation '(designatedfor clarity as operation' "1'Y in `thedrawing)'to about 60% solids to cause the potassiumwcarbonate to crystallize. The liquid phase, 6o taining a higher proportion of potassium chloride vto potassim carbonate than the solid phase, is separated fromme slurryby 'decantatiom leaning a solid'phase cornprised f ot z'ssiumca'rbonatel andl to which isV occluded as "aliquid less than about 1% potassium chloride. i i The solid phase'potassium carbonate is further puriiied byslurrying Viri an aqueous solution'substantially saturated with potassium carbonate; the resulting slurry preferably-domains about"30% undissolved solids by Weight. The'slury'iis'subjected to centrifugation, 2, yielding a liquid rvphase containing about '90% of the potassium chloride originally `contained in the slurry, an'd'a solid phase comprised 'essentially' of "potassium carbonarteQcontaining` lessY than about l0.1% potassium chloride, dry basis. 'lhe liquid phase is recycled to the evaporation stpL.,

-The solid phase is againslurried in an aqueous solutionlsubstantially'.saturated"with essentially pure .potass'ium"carbonate. The resulting slurry is centrifuged, 3, separating the "liquid phase, whichis recycled to centrifuging 2. (The solid phase, comprised essentially of potas- `sium carbonate containing less than about0.01% and generally less than about 0.005% potassium chloride, is driedto yield potassium carbonateof high purity.

' The liquid phase decanted from the slurry of evaporation step l' is cooled to about 15 C., during which cooling essentially all of thepotassium chloride and some of the potassium Vcarbonate orystallizes. The resulting slurry" is .subiected't aY ystwdard flotation in a Denver ilotatyion cell," whereby the potassium chloride crystals arefselectiy'elygoted Vfrom the kpotassium carbonate crysfais." VThe sinkfraetion, comprising potassium carbonate of Vsubstantially reduced potassium chloride content, is recycled tothe evaporation step, 1. -The tloat fraction istransferred at intervals from the trough of the dotation c'ell to aV container wherein the solids are allowed to settle out. VThe transfer of the rst portion of the float product is eifected with Water, but the subsequent portions of the float product are transferred by recycling liquid phase 'from the earlier portions. When the iloat product liquid phase approaches saturation with potassium carbonatefthe entire il'oat'product is Withdrawn and iiltered, thus eticctingseparation of the solid potassium chloride, which is recycled to the potassium hydroxide electrolytic process. The liquid phase, essentially saturated with potssiumcarbouate and containing less than about 1% potassium chloride, is recycled to the potassium carbonate evaporation step 1.

The present invention may be more fully understood fromV` the "following specific example, to be read in conjunction vvith theattached iiowsheet, illustrating a continuous process Vdesigned to produce and purify 1450 pounds of potassium carbonate per hour.

Example An aqueous 45% potassium hydroxide solution weighing`2600 pounds was treated with an excess of flue gas to produce 2890 pounds of a nearly saturated potassium carbonate solution containing approximately k1% portassiurnchlorideasimpurity. 4This solution was combined with recycle solutions, to be described hereinafter, of varying concentrations containing potassium carbonate and 'about l1% potassium chloride. The resulting mixture, totaling 10,915 pounds and containing 98 pounds potassium chloride, was evaporated atabout 95 C., causing potassium carbonate to crystallize, vuntil thesolid phase, constituted about60% of the mixture.V The liquid the receiving tank.

phase, 3200 pounds, containing 48 pounds of potassium chloride, was` separatedV from the solid phaseby decantation While still above C., .and was cooled to 15 C., whereby potassium chloride' and a small proportion of potassium carbonate crystallized. The resulting slurry was subjectedto astandardotation operation in a Denver flotation cell. 41.6% of the potassium chloride present was selectively oated, whereas only 6.0% of the potassium carbonate floated.

Though the entire process was otherwise carried out on a continuous basis, treatment of the vfloat fraction (200 pounds) was effected' in"a`batcli"oper`atiori'l The first float fraction was skimmed oi and Washed with 30 pounds of water down a chute into a receiving tank. A portion of the solid potassium carbonate present in the tloat fraction 'was dissolved in the water, thereby yielding a dilute solution of potassium carbonate .as supernatant liquid in YThis liquid was decanted and used to Wash thenext iioat fraction down the chute, thereby increasing the potassium carbonate concentration of the liquid phase in the receiving tank. This was continued until the liquid phase was nearly saturated with potassium carbonate. The mixture in'thereceiving tank was then iiltered'to separate the solid potassium chloride from the aquetmsv solution of potassium carbonate. The hsolid phase was recycled to electrolysis for potassium hydroxide formation. The liquid phase was recycled to the evaporation step 1.'

VFrm the dectantation step, the slurry phase (5,980 pounds) containing 4020 pounds K2CO3, 50 pounds KCl, and v1910 pounds of Water Was pumped to centrifuge 2 wherein the slurry was spun for 8 seconds, rinsed for .2 seconds with a recycle solution saturated with potassium carbonate, and spun again for 12 seconds. The solid, comprising potassium carbonate With,0.04% KCI, was dnloadedwith the aid'of a recycle solution usaturated with potassium carbonate to a slurry'tanlt'wherein the mixture Nvas thoroughly agitated;` The mixture was then pumped to centrifuge 3 wherein essentially the same procedureas described 'above ivascarried out. The purified s'olidpotass'ium`carbonate from centrifuge 3 was dried, yielding l450""pounds of potassium carbonate containing 0.0034% potassium chloride.V l' 'Y While the invention has been described with reference to certain speciiic details, it isl to be understood that `such details are illustrative only, and are not intended .to be limiting. Many adaptations and modifications of the invention will be apparent from the foregoing description to those skilled in the art.

The following claimsparticularly specify and distinctly claim the subject matterof the present invention:

l. `An improved process for the purification of potassium carbonate'containing potassium chloride as an impurity which comprises preparing a slurry `(I) of said impure potassium carbonate in an aqueous medium selected from the group 4consisting of Water and 'aqueous solutions of potassium carbonate, separating the liquid phase (IDfrorn the solid phase (III) of said slurry (I) ata temperature above about 60 C., cooling said liquid phase (II) to a temperature sutlciently low to form a substantial amount of potassium Vchloride crystals and potassium carbonate crystals therefrom, subjecting the resultant slurry (EV) of potassium chloride and potassium carbonate to dotation, whereby potassium chloride is Vselectively floated therefrom, withdrawing the iloat product, recycling the dotation underflow to said slurry (I) lot' impure potassium carbonate in said aqueous medium, slurrying said solid phase (Ill) with an aqueous solution of essentially pure `potassium carbonate, separatingthe resulting slurryKV) into a liquid phase and'asolid phase (VI), said solid phase (Yi) beingpuriied potassium carbonate of reduced chloride content.

V2.4251 process as in claim 1 wherein the liquid phase (Il) is separated from the solidphase (-111) of slurry (I) at a temperature above about 80 C.

3. A process as in claim 1 wherein said liquid phase (Il) is cooled to about 15 C.

4. A process according to claim 1 wherein said solid phase (Vi) is slurried with an aqueous solution of substantially pure potassium carbonate, and the resulting slurry is subjected to centrifugation to separate the liquid phase from the solid phase, said solid phase being purified potassium carbonate of less than about 0.01% chloride content.

5. In a process for the separation of nely divided, crystalline potassium chloride from admixture with iinely divided, crystalline potassium carbonate in an aqueous medium saturated therewith, the improvement which comprises subjecting said mixture to otation in the absence of a conditioning reagent, whereby said potassium chloride is selectively tloated therefrom, and separating and withdrawing the float product.

6. In a process for the preparation of potassium carbonate, which process comprises passing flue gas through an aqueous solution of potassium hydroxide containing potassium chloride as an impurity, concentrating the resulting aqueous solution whereby potassium carbonate containing potassium chloride as an impurity is obtained as a solid phase (I) in aqueous slurry (II), the improvement which comprises segregating from said slurry (il), at an undissolved solids content greater than about 30% and at a temperature above about 60 C., said solid phase (I) and a liquid phase (I1-I), cooling said liquid phase (III) to a temperature below about 30 C., thereby crystallizing a substantial proportion of the potassium chloride therein as potassium chloride crystals and a quantity of the potassium carbonate contained therein as potassium carbonate crystals, subjecting the resulting slurry (iV) to flotation, whereby potassium chloride is selectively rlioated therefrom, withdrawing the oat product,

recycling the flotation underflow to said slurry (II), slurrying said solid phase (I) with a substantially saturated aqueous solution of essentially pure potassium carbonate, centrifugally separating the resulting slurry (V) into a liquid phase (VI), recycling said liquid phase (VI) to said slurry (Il), and a solid phase (VII), said solid phase (VII) being purilied potassium carbonate of reduced chloride content.

7. A process as in claim 6 wherein the float product is collected and allowed to stratify, the liquid phase is decanted therefrom and used for transferring further quantities of said float product to the collection vessel, said decantation and transfer are continued until said liquid phase approaches saturation with potassium carbonate, the float product is then separated into a solid phase comprising largely potassium chloride and a liquid phase comprising potassium carbonate saturated with potassium chloride, and said liquid phase is recycled to the original aqueous slurry of impure potassium carbonate.

8. A process as in claim 6 wherein said purified potas'- sium carbonate of reduced chloride content is slurried with an aqueous solution substantially saturated with essentially pure potassium carbonate, the resulting slurry is centrifugally separated into a liquid phase and a solid phase, said solid phase being purified potassium carbonate of less than about 0.005% chloride content.

References Cited in the lile of this patent UNITED STATES PATENTS Re. 18,393 Pike Mar. 22, 1932 2,343,080 Pike Feb. 29, 1944 2,448,191 Pike Aug. 31, 1948 2,588,443 Weinig Mar. 11, 1952 2,699,377 Dancy Ian. 11, 1955 2,842,489 Svanoe IJuly 8, 1958 

1. AN IMPROVED PROCESS FOR THE PURIFICATION OF POTASSIUM CARBONATE CONTAINING POTASSIUM CHLORIDE AS AN IMPURITY WHICH COMPRISES PREPARING A SLURRY (I) OF SAID IMPURE POTASSIUM CARBONATE IN AN AQUEOUS MEDIUM SELECTED FROM THE GROUP CONSISTING OF WATER AND AQUEOUS SOLUTIONS OF POTASSIUM CARBONATE, SEPARATING THE LIQUID PHASE (II) FROM THE SOLID PHASE (III) OF SAID SLURRY (I) AT A TEMPERATURE ABOVE ABOUT 60* C., COOLING SAID LIQUID PHASE (II) TO A TEMPERATURE SUFFICIENTLY LOW TO FORM A SUBSTANTIAL AMOUNT OF POTASSIUM CHLORIDE CRYSTALS AND POTASSIUM CARBONATE CRYTALS THEREFROM, SUBJECTING THE RESULTANT SLURRY (IV) OF POTASSIUM CHLORIDE AND POTASSIUM CARBONATE TO FLOTATION, WHEREBY POTASSIUM CHLORIDE IS SELECTIVELY FLOATED THEREFROM, WITHDRAWING THE FLOAT PRODUCT, RECYCLING THE FLOATATION UNDERFLOW TO SAID SLURRY (I) OF IMPURE POTASSIUM CARBONATE IN SAID AQUEOUS MEDIUM, SLURRYING SAID SOLID PHASE (III) WITH AN AQUEOUS SOLUTION OF ESSENTIALLY PURE POTASSIUM CARBONATE, SEPARATING THE RESULTING SLURRY (V) INTO A LIQUID PHASE AND A SOLID PHASE (VI), SAID SOLID PHASE (VI) BEING PURIFIED POTASSIUM CARBONATE OF REDUCED CHLORIDE CONTENT. 